The removal of toxic, corrosive and odorous gases can be accomplished by a number of techniques. These may include wet scrubbing, incineration, and removal via gas-phase air filtration using a variety of dry scrubbing adsorptive, absorptive, and/or chemically impregnated media. As opposed to these other methods, gas-phase air filtration does not require the consumption of large quantities water or fuel. Dry-scrubbing media can be engineered from a number of common adsorbent materials with or without chemical additives for the control of a broad spectrum of gases or tailored for specific gases.
In contrast to the reversible process of physical adsorption, chemical adsorption, also referred to as chemisorption, is the result of chemical reactions on the surface of the media. This process is specific and depends on the physical and chemical nature of both the media and the gases to be removed. Some oxidation reactions can occur spontaneously on the surface of the adsorbent, however, a chemical impregnant is usually added to the media. The impregnant imparts a higher contaminant removal capacity and can lend some degree of specificity. Although some selectivity is apparent in physical adsorption, it can usually be traced to purely physical, rather than chemical, properties. In chemisorption, stronger molecular forces are involved, and the process is generally instantaneous and irreversible.
Undesirable airborne compounds, including sulfur compounds, such as hydrogen sulfide and dimethyl sulfide, ammonia, chlorine, formaldehyde, urea, carbon monoxide, oxides of nitrogen, mercaptans, amines, isopropyl alcohol and ethylene, occur in a number of environments, where most are primarily responsible for the presence of disagreeable odors, or irritating or toxic gases. Such environments include petroleum treatment and storage areas, sewage treatment facilities, hospitals, morgues, anatomy laboratories, animal rooms, and pulp and paper production sites, among others. These undesirable compounds may be bacterial breakdown products of higher organic compounds, or simply byproducts of industrial processes.
Hydrogen sulfide (“H2S”), a colorless, toxic gas with a characteristic odor of rotten eggs, is produced in coal pits, gas wells, sulfur springs, and from decaying organic matter containing sulfur. Controlling emissions of this gas, particularly from municipal sewage treatment plants, has long been considered desirable. More recently, protecting electronic apparatus from the corrosive fumes of these compounds has become increasingly important. Furthermore, H2S is flammable.
Ammonia (“NH3”) is also a colorless gas. It possesses a distinctive, pungent odor and is a corrosive, alkaline gas. The gas is produced in animal rooms and nurseries, and its control also has long been considered important.
Chlorine (“Cl2”) is a greenish-yellow gas with a suffocating odor. The compound is used for bleaching fabrics, purifying water, treating iron, and other uses. Control of this powerful irritant is necessary for the well-being of those who work with it or are otherwise exposed to it. At lower levels, in combination with moisture, chlorine has a corrosive effect on electronic circuitry, stainless steel and the like.
Formaldehyde (“OCH2”) is a colorless gas with a pungent, suffocating odor. It is present in morgues and anatomy laboratories, and because it is intensely irritating to mucous membranes, its control is necessary.
Urea (“OC(NH2)2”) is present in toilet exhaust and is used extensively in the paper industry to soften cellulose. Its odor makes control of this compound important.
Carbon monoxide (“CO”), an odorless, colorless, toxic gas, is present in compressed breathing air. Oxygenation requirements for certain atmospheres, including those inhabited by humans, mandate its control.
Oxides of nitrogen, including nitrogen dioxide (“NO2”), nitric oxide (“NO”), and nitrous oxide (“N2O”), are compounds with differing characteristics and levels of danger to humans, with nitrous oxide being the least irritating oxide. Nitrogen dioxide, however, is a deadly poison. Control of pollution resulting from any of these oxides is desirable or necessary, depending on the oxide.
Mercaptans and amines, including methyl mercaptan (“CH3SH”), butyl mercaptan (“C4H9SH”) and methyl amine (“CH3NH2”), are undesirable gases present in sewerage odor. The control of these gases is desired for odor control.
Isopropyl alcohol (“(CH3)2CHOH”) is a flammable liquid and vapor. Inhalation of the vapor is known to irritate the respiratory tract. Furthermore, exposure to high concentrations of isopropyl alcohol can have a narcotic effect, producing symptoms of dizziness, drowsiness, headache, staggering, unconsciousness and possibly death. The control of this vapor in print processing and industrial synthesis is desired.
Ethylene (“C2H4”) is a colorless, flammable gas. It is a simple asphyxiant that accelerates the maturation or decomposition of fruits, vegetables, and flowers. Control of this compound prolongs the marketable life of such items.
The airborne compounds described above can have a detrimental effect on the local environment. For example, acidification is caused by emissions of sulfur dioxide and nitrogen compounds (nitrogen oxides and ammonia), which in turn cause acid rain. Furthermore, nitrogen oxides and volatile organic compounds from vehicular traffic, electricity and heat production, as well as from industrial facilities may, under certain conditions, contribute to the formation of photochemical oxidants, among which ozone is the dominating substance. Ozone is a colorless gas that forms when nitrogen oxides mix with hydrocarbons in the presence of sunlight. In addition to causing environmental damage, ozone poses a health hazard, particularly for children, the elderly and individuals with asthma or lung disease.
Solid filtration media for removing the undesirable compounds described above are known. Generally described, the filtration media contain a substrate impregnated with high levels of permanganate. The permanganate is typically a permanganate salt such as sodium permanganate (“NaMnO4”), magnesium permanganate (“Mg(MnO4)2”), calcium permanganate (“Ca(MnO4)2”), barium permanganate (“Ba(MnO4)2”) and lithium permanganate (“LiMnO4”).
The substrate is typically formed from one or more of the following: activated alumina, silica gels, zeolites, adsorbent clays and activated bauxite. A preferred porous substrate is alumina. Preferably, the concentration of substrate in the filtration media is from about 40 to 80%, and most preferably is from about 60 to 75% in the absence of sodium bicarbonate and from about 40 to 60% if the media contain sodium bicarbonate.
Another preferred porous substrate is a combination of alumina and a zeolite, in which up to about 50% by weight of the porous substrate combination is a zeolite. Though not intending to be bound by this statement, it is believed that zeolites further control the moisture content of the filtration media by attracting and holding water, which functions to keep more of the impregnate in solution. This effect, in turn, is believed to enhance the high capacity and improved efficiency of the filtration media. As used herein, the term zeolite includes natural silicate zeolites, synthetic materials and phosphate minerals that have a zeolite-like structure. Examples of zeolites that can be used in this media include, but are not limited to, amicite (hydrated potassium sodium aluminum silicate), analcime (hydrated sodium aluminum silicate), pollucite (hydrated cesium sodium aluminum silicate), boggsite (hydrated calcium sodium aluminum silicate), chabazite (hydrated calcium aluminum silicate), edingtonite (hydrated barium calcium aluminum silicate), faujasite (hydrated sodium calcium magnesium aluminum silicate), ferrierite (hydrated sodium potassium magnesium calcium aluminum silicate), gobbinsite (hydrated sodium potassium calcium aluminum silicate), harmotome (hydrated barium potassium aluminum silicate), phillipsite (hydrated potassium sodium calcium aluminum silicate), clinoptilolite (hydrated sodium potassium calcium aluminum silicate), mordenite (hydrated sodium potassium calcium aluminum silicate), mesolite (hydrated sodium calcium aluminum silicate), natrolite (hydrated sodium aluminum silicate), amicite (hydrated potassium sodium aluminum silicate), garronite (hydrated calcium aluminum silicate), perlialite (hydrated potassium sodium calcium strontium aluminum silicate), barrerite (hydrated sodium potassium calcium aluminum silicate), stilbite (hydrated sodium calcium aluminum silicate), thomsonite (hydrated sodium calcium aluminum silicate), and the like. Zeolites have many related phosphate and silicate minerals with cage-like framework structures or with similar properties as zeolites, which may also be used in place of, or along with, zeolites. These zeolite-like minerals include minerals such as kehoeite, pahasapaite, tiptopite, hsianghualite, lovdarite, viseite, partheite, prehnite, roggianite, apophyllite, gyrolite, maricopaite, okenite, tacharanite, tobermorite, and the like.
Processes for making a dry-scrubbing media composition are known. For example, a mixture of activated alumina, magnesium oxide and a liquid can be formed into at least one cohesive unit, and the cohesive unit cured at an elevated temperature, preferably 100-225° F. for at least one hour. Preferably, a dry feed mix is made of the activated alumina and magnesium oxide, and the dry feed mix is tumbled or rolled while being sprayed with a liquid, for example water. The dry feed mix may further include powdered activated carbon.
Heating the impregnating solution prior to rolling the pellets in a tumble mill appears to allow the pellets to begin curing immediately, yielding better physical characteristics than an impregnating solution applied at room temperature. This can be achieved using a solution temperature between about room temperature and the boiling point of the solution. A preferred solution temperature is about 50° F. to about 200° F.
The amount of moisture present in the composition will depend on several factors, related primarily to the characteristics of the activated alumina being treated. The desired moisture content of the composition is readily obtained by spraying the dry mix ingredients while they roll on the mixer, in accordance with the method of U.S. Pat. No. 3,226,332, the entire contents of which are incorporated herein in their entirety.
Impregnation of the dry scrubbing media may be carried out in any manner which effectively produces an adsorbent of about 0.1% to about 15% by weight of impregnate formed by using a solution of about 0.3% to about 40% impregnate. Impregnation may be carried out simply by soaking the combinations in one volume of impregnate solution. The time required to produce the desired impregnation level is dependent upon the impregnate employed, and will only be as much time as is needed for the impregnate to penetrate the combinations. Additionally, the impregnate solution may be heated prior to use, for example during preparation of a dry-mix or during a tumbling/rolling process.
For example, impregnation with a hydroxide may be carried out by using a solution of about 3% to about 20% sodium hydroxide or potassium hydroxide. The resulting pellet should contain from about 1% to about 10% by weight hydroxide. Impregnation with other suitable impregnates also may be carried out in any manner that effectively produces an adsorbent of about 1% to about 10% by weight of impregnate, formed by using a solution of up to about 40% impregnate.
Alternatively, the impregnate solution may be passed through the media rather than being used in a static immersion treatment.
Additionally, the dry-scrubbing media may be formed by extrusion to form a matrix or honeycomb structure. The formation of channels and pores in a matrix creates a large surface area for chemical reactions to occur between contaminants in an air-stream and the surface of the dry-scrubbing media.
Such media, and methods for making the media, are described in several patent publications, including International application publication Nos. WO 2004/047950 and WO 2008/067521 and U.S. patent application publication No. 2009/0246107 the entire contents of which are incorporated herein in their entirety.
The methods for making the media described above and in the references incorporated herein result generate a significant amount of waste. For example, fines, oversized media, and waste from the equipment used to form the media (such as extruders and pelletizers) can account for waste of up to 10% or more as compared to the starting materials.
This waste media, because it has been treated with water during the manufacturing process, has already gone through the hydration process and has been inactivated. As a result, it cannot be re-used in the pelletization or extrusion process because it will not stick together as a cohesive unit. The waste media could be regenerated by heating it in a kiln, but this is a costly step that is typically not carried out in commercial processes. Accordingly, this waste media is typically disposed of in landfills. The cost of the unusable/wasted media and the costs for transporting and disposing of the waste media in a landfill can account for hundreds of thousands of dollars in annual loss in large-scale production facilities.
For at least these reasons, it would be desirable to have a low-cost option for re-using the waste media generated during these processes so as to minimize the need to send this waste to a landfill.